Liquid crystal displays are widely used in personal computers, mobile equipment monitors and televisions since they have various advantages, e.g., in their low voltage and low consumption power and high possibility for reduction in size and profile. Although a variety of modes depending on how liquid crystalline molecules are aligned in a liquid crystal cell have been proposed for such liquid crystal displays, the dominating mode has hitherto been a TN mode in which liquid crystalline molecules are in an aligned state that their orientations twist by about 90° toward an upper side substrate from a lower side substrate.
In general a liquid crystal display is made up of a liquid crystal cell, an optical compensation film and a polarizer. The optical compensation film is used for dissolution of coloring of images and expansion of a viewing angle, and a stretched birefringent film or a transparent film coated with a liquid crystal is employed as the optical compensation film. For instance, Japanese Patent No. 2587398 discloses the art of expanding a viewing angle by applying to a TN-mode liquid crystal cell the optical compensation film formed by coating a discotic liquid crystal on a triacetyl cellulose film, forcing the liquid crystal into an aligned state and fixing the aligned state. However, liquid crystal displays for television use, which are supposed to be equipped with big screens and to be viewed from various angles, have stringent demands on viewing angle dependence, so even the foregoing art cannot satisfy such demands. Under these circumstances, liquid crystal displays employing modes different from the TN mode, such as an IPS (In-Plane Switching) mode, an OCB (Optically Compensatory Bend) mode and a VA (Vertically Aligned) mode, have been studied. The VA mode in particular has captured the spotlight in liquid crystal displays for TV uses because it can ensure high contrast and relatively high manufacturing yield.
Cellulose acylate films have a feature that they are high in optical isotropy (low in retardation value), compared with other polymer films. Accordingly, it is a general rule that cellulose acetate film is used for applications requiring optical isotropy, such as for polarizing plates. JP-A-2000-131524 in particular discloses the method of manufacturing a cellulose acetate film having high transparency and a low content of insoluble matter by specifying the relationship between a viscosity-average polymerization degree of cellulose acetate and a viscosity of the dope prepared by dissolving the cellulose acetate in a solvent. In addition, JP-A-2001-129838 discloses a desirable relationship between the thickness d of cellulose acetate film, the solids content y (%) in a solution for forming the cellulose acetate film and the viscosity ρ of the solution for the purpose of resolving sheet troubles referred to as die streaks.
By contrast, optical anisotropy (high retardation value) is required of optical compensation sheets (retardation films) used in liquid crystal displays. The optical compensation sheets for VA-mode in particular are required to have an in-plane retardation (Re) of 30 nm to 200 nm and a thickness-direction retardation (Rth) of 70 nm to 400 nm. Therefore, it is a general rule that synthetic polymer films having high retardation values, such as polycarbonate film and polysulfone film, are used as optical compensation sheet.
As mentioned above, it has been a general rule in the technical field of optical materials that synthetic polymer films are used in the case of requiring for polymer films to have optical anisotropy (high retardation values Re and Rth), whereas cellulose acetate film is used in the case of requiring for polymer films to have optical isotropy (low retardation values).
EP-A-911656 discloses the cellulose acetate film having high retardation values which, though against the rule hitherto regarded as general, is also usable for applications requiring optical anisotropy. In that patent, a compound having at least two aromatic rings, notably a compound having a 1,3,5-triazine ring, is added and stretch processing is performed in order to achieve high retardation values in the case of using cellulose acetate.
Although it is generally known that cellulose acetate is a polymer material hard to stretch and its birefringence factor is difficult to increase, the patent document cited makes it possible to increase the birefringence factor through simultaneous alignment of the additive molecules by stretch processing and achieves high retardation values (Re, Rth). Such a film can also serve as protective film of a polarizing plate, so it has an advantage in its suitability for offering thin liquid crystal displays at low prices.
The method described in the patent document cited helps offer thin liquid crystal displays at low prices.
On the other hand, when liquid crystal displays are exposed to varying temperatures and humidities, changes in dimensions are induced in polarizing plates and thereby glass cells suffer warping. When the glass cells suffer warping, the edge part thereof is brought into contact with a flame part and light leaks occur in the contact part. A main cause of the warping is dimensional changes occurring in the polarizing plate through shrinkage in its polarizer. Chief among measures to control the warping is enhancement of stiffness of a protective film for the polarizer, e.g., through an increase in thickness or elasticity modulus of the protective film. The term “stiffness” as used herein is defined as the product of film thickness and elasticity modulus. Since thickness reduction is now required for polarizing plates, it is impossible to increase the thickness in the extreme. Therefore, films with high elasticity moduli have been required. With respect to the control of film's physical properties, however, controls of an in-plane retardation and a thickness-direction retardation, which relate to display qualities, such as viewing angle contrast and hue, have been assigned the highest priority, whereas elasticity modulus control has been difficult since even any technique to control an elasticity modulus has never been developed.
The light leaks are observed more noticeably when there is a large difference between the elasticity modulus in a film-conveying direction (machine direction) and the elasticity modulus in the direction orthogonal thereto.
This phenomenon stems from anisotropy induced in amounts of dimensional changes depending on temperature and humidity. In regard to this point also, it has been expected to initiate improvements.